Pavement Design.docx

Types of pavement structures

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Pavements, in general, can be classified in two major categories: concrete pavement and bituminous pavement. Concrete pavements are generally called rigid pavements and bituminous pavements as flexible pavements. There could be some other types of pavements which are neither rigid, nor flexible, for example, block pavement, composite pavement. A Pavement is a multi-layered structure. The layers are placed one over other. In general, the strengths of the layers decrease from top towards bottom except some special situation like inverted pavement. The terminologies used to identify various layers of bituminous and concrete pavements are identified in Figs. 1 and 2

. Fig. 1 cross section of a typical bituminous pavement (Chakroborty and Das 2003)

Fig. 2 cross section of a typical rigid pavement (Chakroborty and Das 2003) Bituminous pavement

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The subgrade is a compacted soil layer. The base and sub-base course could be made up of bound or unbound granular layer. As per Indian specifications (MORT&H 2001), some examples of base or sub-base layers are: Granular sub-base(GSB), Water Bound Macadam (WBM), Wet Mix Macadam (WMM) etc. The binder course is made up bituminous material. As per Indian specifications (MORT&H 2001), some examples of binder course are: Bituminous Macadam (BM), Dense Bituminous Macadam (DBM) etc. The wearing course is the top bituminous layer which is comes in contact to the vehicle tyre. Wearing course provides impermeability to the pavement surface against water percolation (Chakroborty and Das 2003). The binder course and wearing course together are called bituminous surfacing.

Concrete pavement

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Concrete pavement is, in general, consists of three layers, subgrade, base layer and the concrete slab. Generally bound base layers are used for concrete pavement construction. As per Indian specification, some example of such base layers are Dry Lean Concrete (DLC), Roller Compacted Concrete (RCC) (IRC:15-2002) The concrete slab is generally of M40 to M50 grade of concrete as per Indian specifications, and is called as paving quality concrete (PQC) (IRC:15-2002).

Joints in concrete pavement

Fig.3 Location of joints in concrete pavement

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Joints are the discontinuities in the concrete pavement slab, and help to release stresses due to temperature variation, subgrade moisture variation, shrinkage of concrete etc. There are various types of joints in concrete pavement, e.g. contraction joint, construction joint, expansion joint and warping joint. Fig. 3 schematically shows position of various joints. The functions of these joints are as follows:  Contraction joint: Contraction joints are provided along the transverse direction to take care of the contraction of concrete slab due to its natural shrinkage.  Construction joint: Construction joints are provided whenever the construction work stops temporarily. The joint direction could be either along the transverse or longitudinal direction.  Expansion joint: Expansion joints are provided along the transverse direction to allow movement (expansion/ contraction) of the concrete slab due to temperature and subgrade moisture variation.  Warping joint: Warping joints are provided along the longitudinal direction to prevent warping of the concrete slab due to temperature and subgrade moisture variation. These discontinuities (joints) could be extended to the full or partial depth of the slab. Sometimes iron bars are provided across the joints, the iron bars along the longitudinal joints are called tie bars andalong the transverse joints are called dowel bars.

Pavement analysis and design: historical perspective

The past pavement design approaches were mostly empirical in nature and were based on experience. 

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CBR method of pavement design is one of the earlier pavement design approach developed during 1928-29 (Ullidtz 1986). In this method the thickness design charts are developed (based on observation of number of sections), with reference to the subgrade CBR value for the most critical moisture condition. In 1940 this method was adopted by the U.S. Corps of Engineers for design of airfield pavements (Horonjeff and Mckelvey 1983). It is interesting to note that the design initially did not involve traffic as a parameter, which was introduced later as a correction factor. The method was further improved by considering the CBR values of the individual layers and thereby individual layer thicknesses are obtained. In some other approach, Hveem resistance value of pavement materials is used instead of CBR value. Another pavement design approach considers aspect of bearing capacity of the individual layers, and the design is finalized in such a way that the bearing stress does not exceed the bearing capacity of the individual layers. This method was first proposed by Barbar in 1946, and is still in use (TRH4 1996, deBruin et al. 2002), however this method does not seem to account for traffic repetitions. Another approach recommends limiting recoverable deflection as the criterion for pavement design (Huang 1993). Failure theories suggest that the failure of a structure is due to excess stress or strain, thus, deflection may not be attributed as basic pavement design criteria.

Pavement analysis and design: current perspective

Present practice of pavement design involves considerations of three aspects: structural design, functional design and drainage design and they are explained briefly in the following: Structural design

In structural design the stresses due traffic loading and temperature are estimated, and the thickness of the pavement is designed in such a way that these developed stresses/ strains are below the allowable values. The current practice of pavement design, more popularly, is known as Mechanistic-Empirical pavement design and is followed by a number

of organizations around the world (Asphalt Institute 1999, Shell 1978, Austroads 1992, NCHRP 2005, IRC 2001). It is mechanistic pavement design because it uses stress/ strain of a pavement structure using mechanics based principle, and, as well, it is empirical because the expected life for a given stress/ strain level is estimated from empirical relationships obtained from laboratory or field performance studies. The pavement design approach is not governed by the maximum amount of load that the pavement can sustain, rather, it estimates the number of standard load repetitions that can cause failure. Estimation of pavement stress/strain Stress/ strain due to load

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For pavement design purpose, the stress/ strain value of a pavement structure is obtained from structural analysis of the pavement (Ioannides et al. 1998). The stress/ strain values at any point of a pavement structure can be estimated when the elastic moduli, Poisson's ratio and the thicknesses of the individual layers are known. The strain values can also measured using strain gauges. Any analysis procedure involves idealization regarding the structure; similarly, measuring strain involves measurement errors - hence the true value of stress/ strain is never known. A concrete pavement slab, in general, has finite dimensions, and thus the analysis approach of concrete pavement becomes different than the analysis of bituminous pavement. For bituminous pavement, in general, the pavement is assumed as infinite in both the directions, whereas for concrete pavement, in general, it is analysed as discrete slabs connected by joints. The concrete pavement is also assumed to have bending moment carrying capacity, whereas flexible pavement is assumed to have no moment carrying capacity.

Stress/ strain due to temperature

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The change of temperature causes the pavement to expand or contract. The restriction of free movement causes temperature stresses. There exists temperature variation across the depth of the pavement - this causes warping stresses. The temperature stress varies across the corner, interior and edge of the concrete slab, also at different times of the day. The most critical combination of load and temperature stress is used as design criteria. The temperature stress in bituminous pavement is insignificant. Hence, temperatire stress, is not considered in pavement design. However, temperature affects the elastic modulus of the bituminous layer, which needs to be duly considered in pavement design.

Estimation of layer thicknesses

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The thickness of the pavement is adjusted in such a way that the stress/ strain developed is less than the allowable values obtained from past performance information. The two major modes of structural failure of pavement are fatigue and rutting. o Fatigue: Traffic applies repetitive load to the pavement surface, and the cracks start from bottom the bound layer/ slab and propagate upwards. When the extent of surface cracks reaches a predefined level, the pavement is said to have failed due to flexural fatigue.

. 4 Propagation of fatigue cracking

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The horizontal tensile stress/ strain at the bottom of bound layer (bituminous surfacing, cemented base or concrete slab, as the case may be) is used as the governing parameter for fatigue failure. o Conventionally, for design of concrete pavement stress is used as parameter, and for design of bituminous pavement strain is used as parameter. Rutting: Rutting is the accumulation of permanent deformation. This is the manifestation of gradual

Fig 5. Development of Rutting

The vertical strains on the pavement layers, mainly the vertical strain on the subgrade is assumed to be governing factor for rutting failure.  

o The rutting is generally not considered for concrete pavement design. The fatigue/ rutting equations are developed from field or laboratory studies, where fatigue / rutting lives are obtained with respect to respective stress/ strain for fatigue/ rutting. For a given design life, thus, allowable fatigue and rutting stress/ strains can be estimated using the fatigue/ rutting equations. The various other types of pavement failures could be shrinkage, thermal fatigue, top down cracking (for bituminous pavement) etc.

Design of joints

The spacing of the contraction joint is estimated from the shrinkage potential of concrete. The spacing of the expansion joint is estimated from the coefficient of thermal expansion of concrete, maximum change of temperature and the acceptable joint gap. Since, the concrete is good in compression, the experience over last few decades indicates that concrete pavement can be constructed without any provision of expansion joint (ACPA 1992). The dowel bars are designed by assuming that they participate in the load transfer, when the vehicle moves from one slab to other. The tie bars are designed in such way that they have enough strength to tie the two adjacent slabs. The design of dowel bar and tie bar is discussed in detail later. Functional design

The functional pavement design involves considerations of skid resistance, roughness, surface distresses, reflectivity of pavement surface etc. The functional pavement design considers mainly the surface features of a pavement. Drainage design

A road needs to be designed in such a way that the rain/ snow precipitation is drained off the pavement and its surroundings. A suitable surface drainage system for the pavement is designed for this purpose. Some water, however, will percolate into the pavement from its top surface and needs to be taken out of the pavement - this is done by providing an internal drainage system to the pavement. Water will also try to enter into the pavement from bottom due to capillary rise or due to rise in water table. A suitably designed sub-surface drainage system tries to avoid such a problem. Pavement analysis and design: future perspective

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Mix design, quality control, construction method and pavement design together determines the performance and longevity of a pavement. The future pavement design is expected to take an integrated design approach considering all these issues together. The parameters associated with pavement design are stochastic in nature. Thus, the two pavement designs (designed deterministically) having same design traffic may have different levels of reliabilities of survival. Thus, reliability issues of pavement design are gradually becoming important considerations (NCHRP 2005). A pavement designer essentially looks for the most economical design, yet considering the structural, functional, and drainage design requirement. The future pavement design practice is expected to consider the cost optimality over the entire life cycle of the pavement (Deshpande etal. 2010).

Recapitulation

The present module highlights the basics of analysis and design of bituminous and concrete pavements. Pavements are analysed as layered horizontal structure with given elastic moduli and Poisson's ratio. Concrete pavement, in general, is made up of discrete slabs - therefore, it has joints both in longitudinal and transverse direction. A pavement is designed from structural, functional and drainage considerations. Fatigue and rutting are two major modes of structural failure of pavements. For concrete pavement design, temperature stresses are also considered along with stresses due to load. A pavement designer does not design a pavement for the ultimate load the pavement can carry, rather, the number of repetitions that the pavement can sustain.

PAVEMENT MATERIALS Objectives

The lectures in this module propose to introduce the modern materials in pavement construction. It discusses about the scope, application potential, evaluation, and performance expectation of the new highway materials. The second part of the lectures focus on the innovative application concepts of the conventional or the modern materials. Usage of modern materials in highway construction and their innovative application is expected to bring economy in terms of material cost as well as better reliability in performance. Bitumen as a pavement material

The characterization of bitumen and bituminous mix has been discussed in detail in the web-course Transportation Engineering - I Bitumen is a complex material, its property ranges from viscous liquid to brittle solid. While bitumen shows linear viscoelastic behaviour at small strains, the nonlinear behaviour becomes more prominent at large strains (Monismith and Secor 1962, Pagen 1968, Cheung and Cebon 1997). The deformation of bitumen is loading rate and temperature dependent (Van der Poel, 1955, Deshpande and Cebon 1997). The bituminous mix is manufactured by mixing bitumen and aggregates of specified size distribution at some specified elevated temperature. Then, the mix is transported to the site, laid and subsequently compacted to pack the aggregate particles together. During the compaction process the air voids are brought down to its desired level. The compacted mix, thus, achieves its strength when it cools down and becomes serviceable as bituminous road . Figure-1 shows a typical cross-section of a bituminous mix sample.

Figure-1 A typical cross-section of a bituminous mix sample

The mechanical behaviour of bituminous mix has been studied extensively through various tests, and empirical relationships have been developed for mix design and prediction of the performance of the mix. However, prediction of response of bituminous mix through mechanics based models, is a difficult task. Various attempts have been made by the researchers, for example based on, linear viscoelastic principle (Lee and Kim 1998, Kim and Little 2004), elastic visco plastic principle (Uzan 2005), discrete element analysis (Sadd 2004, Abbas et al. 2005) etc., so as to capture the complex mechanical behaviour of bituminous mix. Cement Concrete as a pavement material Introduction

Cement concrete is a mixture of coarse aggregates, fine aggregates, cement and water in suitable proportions. Sometimes admixtures are also added to achieve specific behaviour/ property of the material. The components of cement concrete are briefly introduced in the following. Components of cement concrete

Aggregates

Aggregates are naturally available pieces of rocks. The aggregates could be igneous, sedimentary and metamorphic type depending on its origin. Figure-1 shows a photograph of aggregates being manufactured from a stone query. The details about the physical properties of aggregates have discussed in the web-course on Transportation Engineering 1.

Figure 2: A typical stone quarry Cement

Cement is manufactured by heating a mixture of limestone, iron ore, gypsum, clay and other ingredients. Two processes, namely dry process and wet process are followed while manufacturing cement. In the dry process, the raw materials are mixed in dry state, whereas in the wet process raw materials are mixed in presence of water to form slurry . After pre-heating, the raw material is passed through rotating kiln inclined with a small angle with the horizontal line. The kiln is progressively hotter towards its lower end, where the raw material gets molten. From this clinkers are formed when cooled, and after grinding the clinkers, cement is produced. An animated description of the whole process can be obtained elsewhere (cement.org 2006). The Ordinary Portland Cement (OPC) is the most popular, all-purpose cement. There are various other types of cements (for example, natural cement, Portland pozzolanic cement, high alumina cement, expansive cement, quick setting cement, high performance cement, sulphate resistant cement, white cement etc.) and are manufactured to serve specialized purposes. For concrete pavement construction, OPC is most commonly used. Water

Water participates in the hydration process; also it provides desirable level of workability. About one third of the water added is utilized in the hydration process, rest forms the pores of concrete, and thereby developing porosity to the concrete. Excess porosity reduces strength of the concrete, and however presence of porosity is good for the situations where there is a freeze-thaw problem. Admixtures

Admixtures are generally of two types, chemical admixture, and mineral admixture. Air entrainer, retarder , accelerators are examples chemical admixture, and, fly ash, silica fume are the examples of mineral admixtures. One of the important concrete admixtures used in pavement construction is the air-entraining admixture. Air entraining admixtures are derived from natural wood resins, fats, sulfonated hydrocarbons and oils etc (Wright and Dixon 2004). Air-entraining admixtures provide durability against freeze-thaw situation. Plasticizers may be used for concrete pavement construction purposes which maintain workability without having increased the watercement ratio. Calcium chloride is also used sometimes, as accelerating agent, which renders an early strength of concrete. Mix design

Through mix design, suitable proportions of the ingredients (coarse aggregates, fine aggregates, cement, water and admixture, if any) are estimated, keeping in view the strength, workability, durability and economic considerations. These proportions are achieved through iterative experimental procedure in the laboratory. There are number of methods for mix design of cement concrete, and a detailed discussion can be obtained elsewhere (Neville and Brooks 1999). Water-cement ratio is an important consideration in the mix design process. As water cement ratio is increased in concrete, the durability and strength decreases, however, the workability enhances. Depending on the type of construction, workability requirements are different. For large scale production of cement concrete, the proportioning operation is performed in the batch mixing plant. Figure 3 shows a photograph of a typical concrete batch mixing plant.

Figure 3: A typical cement concrete batch mixing plant Properties of fresh concrete

Ideally a fresh concrete should be workable, should not segregate or bleed during construction. Constituent properties, their proportions, aggregate shape and sizes, temperature affect the performance of fresh mix. The tests that are conducted on fresh concrete include workability test and air-content test. Some of tests through which workability of can be estimated are Kelly ball penetration test, slump test, compacting factor test, Vee bee test and flow table test etc. Curing of concrete

Presence of adequate amount of moisture, at some requisite temperature and for a suitable period of time, is necessary to complete the hydration process of cement. This process is called curing. The curing conditions significantly affect the final strength achieved by the concrete. For pavement construction, only in-situ curing methods are applicable. Curing compounds are sometimes applied to retain the moisture against evaporation. For final curing of concrete pavements continuous ponding or moistened hessain/ gunny bags are used .

Properties of hardened concrete Tests are conducted on hardened concrete to estimate properties like, compressive strength, tensile strength, modulus of rupture, elastic modulus, Poisson's ratio, creep and shrinkage performance, durability, thermal expansion coefficient etc. These parameters are of functions of aggregate type, shape and size, type and quantity of cement and admixtures incorporated, water cement ratio, curing, age etc. Compressive strength of concrete is the failure compressive stress on cubical or cylindrical samples of concrete. Compressive strength of concrete is related to the combined effect of temperature and time, a parameter called maturity. Maturity of concrete is calculated as the time of curing (in hours), multiplied by the temperature, (in degrees) above some specified reference temperature. Various empirical relationships are suggested to obtain the various strength parameters of concrete (elastic modulus, tensile strength, bending strength etc.) from the compressive strength of concrete. Direct tension test on concrete is performed by applying tension to the cylindrical or dumble shaped samples of concrete. Indirect tension is applied to concrete samples by split cylinder test. Modulus of rupture of concrete is estimated by measuring the maximum bending stress on concrete beam subjected to pure bending in static condition. Fatigue test is generally performed subjecting the concrete beams with repetitive flexural loading. The more is the stress ratio (defined as the ratio between the bending stress applied to the modulus of rupture) the less is the fatigue life. The empirically derived fatigue equation by PCA (1974) is the following: (1) and (2) Where, Nf is the number of load applications to failure, SR is the stress ratio with reference to 90 days modulus of rupture. The equation suggested by AASHTO (1993) is the following: (3) Transportation of concrete The transportation of concrete is to be done in such a way that segregation and premature setting is avoided. Wheel barrow, truck mixer, dumper truck, belt conveyor, pipe-line etc. are the various ways concrete is transported to the construction site. Figure 4 shows a typical truck concrete mixer.

Figure 4: A typical truck concrete mixer

Introduction Road is a costly infrastructure to build and maintain. Thus there is always a need of development of (i) new road materials as well as (ii) innovative applications of existing/new materials (Goel and Das 2004). These issues are discussed here.

EMERGING ROAD MATERIALS Modification of Existing Materials Existing materials may require modifications so as to

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improve engineering properties of material satisfy general specification requirement of locally available material which in turn would prove to be cost effective meet the demand of special purpose materials having specific properties. Example: high or low permeability, enhanced shear strength etc

These have been discussed further under two sections as,

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binder (bitumen) modification aggregate modification

Binder (bitumen) modification

Binder (bitumen) modification is done with the help of additives which may or may not react chemically with bitumen. Table 1 presents a partial list of various types of binder modifiers, their purpose and examples. Binder modification results improvement of one or more properties of the binder (and hence the mix) viz. fatigue resistance, stiffness modulus, rutting resistance, stripping potential, temperature susceptibility, oxidation potential etc. Table 1. Some examples of binder (bitumen) modifiers

(RILEM 1998; ETM 1999; Asphalt Handbook 2000; Widyatmoko 2002, SEAM 2004 ) Purpose

Examples

Non Polymers Fillers

to improve bitumen durability Lime, carbon black, fly ash and check rutting

Anti-oxidants

to check oxidative hardening Zinc anti-oxidants, phenolics, amines

Anti-stripping additives

to achieve better adhesion of Organic compounds bitumen to aggregates andamides)

Extenders

to act as bitumen substitute and to improve fatigue resistance

Lignin, sulphur

to reduce viscosity, as filler material,

Polyester fibers, Polypropylene fibers

lead

anti-oxidant,

(like

arnines,

Polymers Fibers Plastics -Thermoplastics

to increase the viscosity and stiffness of bitumen at normal

service temperatures without Polyethylene (PE), Polypropylene (PP), compromising with fatigue Polyvinyl chloride (PVC), Ethylene vinyl performance acetate (EVA).

-Thermosets

to obtain insoluble, infusible material that do not flow on heating

Epoxy resins

to reduce temperature susceptibility and temperature distresses, agehardening, bleeding and binder-aggregate stripping.

Rubber

3. Elastomers - Natural - Synthetic - Reclaimed rubbers

Styrene-butadiene copolymer (SBR), Styrene-butadiene-styrene copolymer (SBS), Isobutene-isoprene copolymer (IIR)

For conventional binders, it is generally observed that the mixes with high stiffness modulus (E) show low fatigue life, and vice versa. However, for an economical pavement design, both high elastic modulus as well as high fatigue life is desirable. Through binder modification, this particular disadvantage can be avoided. Figure 5 presents this concept schematically. As can be seen in Figure 5, for mixes with ordinary binder, although elastic modulus E value is higher initially at low temperatures, at high E value the fatigue performance generally becomes poor. On the other hand, at high temperature the E value becomes too low and the mix becomes soft. The bituminous mixes with modified binder does not allow the mix to be too hard (high E value) or too soft (low E value) at low and high temperatures respectively. Thus the stiffness versus temperature curve takes a 'S-shape' as shown in Figure 5. Aggregate modification

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The marginal or poor quality aggregates can be improved by using some cementing material such as cement, lime, pozzolanic substance etc. The proportions of the cementing material and other ingredients (like water) can be suitably estimated in the laboratory.

DEVELOPMENT OF ALTERNATIVE MATERIALS     

Given the fact that good quality aggregates are depleting and cost of material extraction is increasing, researchers are looking for suitable alternative materials. The tests and specifications, which are applicable for conventional materials, may be inappropriate for evaluation of non-conventional materials ( i.e. alternative materials). This is because the material properties, for example, particle sizes, grading and chemical structure, may differ substantially from those of the conventional materials. Thus, for an appropriate assessment of these materials, new tests are to be devised and new acceptability criteria are to be formed. However, with the advent of performance-based tests, it is expected that the performances of the conventional as well as new materials can be tested on a same set-up and be compared.

Industrial and Domestic Wastes

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Industrial and domestic waste products provide a prospective source of alternative materials. These materials are cheaply available. Also, their use in road construction provides an efficient solution to the associated problems of pollution and disposal of these wastes.

Table 2 presents a partial list of industrial waste materials that can be used in road construction. Table 3 summarizes the

advantages and disadvantages of using specific industrial wastes in road construction. Table 2. Industrial waste product usage in road construction (TFHRC 2004; Hamad et al., 2003; Hawkins et al., 2003; Mroueh et al., 2002; Okagbue et al. , 1999; Sherwood 1995; Javed et al., 1994)

Waste product

Source

Possible usage

Fly ash

Thermal power station

Bulk fill, filler in bituminous mix, artificial aggregates

Blast furnace slag

Steel industry

Construction and demolition waste

Construction industry

Base/ Sub-base material, Binder in soil stabilization (ground slag) Base/ Sub-base material, bulk-fill, recycling

Colliery spoil Spent oil shale

Coal mining Petrochemical industry

Bulk-fill Bulk-fill

Foundry sands Mill tailings Cement kiln dust

Foundry industry Mineral processing industry Cement industry

Bulk-fill, filler for concrete, crack-relief layer Granular base/sub-base, aggregates in bituminous mix, bulk fill Stabilization of base, binder in bituminous mix

Used engine oil

Automobile industry

Air entraining of concrete

Marble dust Waste tyres

Marble industry Automobile industry

Filler in bituminous mix Rubber modfied bitumen, aggregate

Glass waste Nonferrous slags

Glass industry Mineral processing industry Bricks and tile industry

Glass-fibre reinforcement, bulk fill Bulk-fill, aggregates in bituminous mix

China clay

Bulk-fill, aggregates in bituminous mix

Table 3. Suitability of using industrial waste products in road construction (TFHRC 2004; Hamad et al., 2003; Hawk ins et al., 2003; Nunes et al. 1996; Sherwood 1995; Javed et al., 1994) Material Fly ash

Metallic slag - Steel slag

Advantages Disadvantages Lightweight, used as binder in stabilized base/ sub-base Lack of due to pozzolanic properties homogeneity, presence of sulphates, slow strength development Higher skid resistance Unsuitable for concrete and fill work beneath Light weight ( phosphorus slag) slabs.

- Nonferrous slag

demolition waste

May show inconsistent properties More strength, can be used as aggregates granular base May show inconsistent properties

Blast furnace slag

Used in production of cement, granular fill

Construction and

Ground water pollution due to leachate formation, used as unbound aggregates

Colliery spoil

Spent oil shale

Foundry sands

Mill tailings

Cement kiln dust

Used engine oil

Rubber tires

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Combustion of unburnt coal, sulphate attack in case of concrete roads Burning of combustible materials Substitute for fine aggregate in bituminous mixes Presence of heavy metals in non ferrous foundry origin, less affinity to bitumen Some are pozzolanic in nature Presence of poisonous materials (e.g., cyanide from gold extraction) Hardens when exposed to moisture, can be used in soil Corrosion of stabilization metals (used in concrete roads) in contact because of significant alkali percentage Good air entertainer, can be used in Requires well organized used oil collection concrete works system Enhances fatigue life

The incenerated municipal soild waste (MSW), after further processing, can be used as fines in bituminous mixes. Processing is done to remove ferrous and nonferrous metals and to achieve the required particle size gradation. Due to the presence of larger fraction of fines, MSW ash is primarily used as fine aggregate. It is also used as a fill material in road construction. The ash can also be stabilized with portland cement or lime to produce stabilized base/sub-base material (TFHRC 2004). For conventional road materials, a number of tests are conducted and their acceptability is decided based on the test results and the specifications. This ensures the desirable level of performance of the chosen material, in terms of its permeability, volume stability, strength, hardness, toughness, fatigue, durability, shape,viscosity, specific gravity, purity, safety, temperature susceptibility etc. , whichever are applicable. There are a large number tests suggested by various guidelines/ specifications. Figure-6 presents a suggested flow chart to evaluate the suitability of industrial waste for potential usage in highway construction.

Requires special techniques for fine grinding and mixing with bitumen, sometimes segregation occurs

Figure - 6 Evaluation industrial waste for suitability in highway construction

Health and safety considerations should be given due importance handing industrial waste materials ( Mroueh and Wahlström 2002, Nunes et al. 1996). 1.2.2. Other alternative materials

Steel slag aggregate is a good example of synthetic aggregates obtained from by-products of industrial processes. It has good binding properties with bitumen due to its high calcium oxide content (NatSteel 1993). The angular shape of the aggregates helps to form strong interlocking structure. Road paving with steel slag aggregate show  

good skid resistance mechanical strength able to withstand heavy traffic and surface wearing.

Also, many industrial and other waste products like fly-ash, cement kiln dust, incenerated refuse etc. have been successfully used to produce synthetic aggregates. Mixing bitumen with rubber (natural or crumb form) sometimes poses difficulty. As an alternative approach, tiny crumb rubber pieces can be mixed with aggregates - known as dry-process. Research shows improved fatigue performance for this kind of materials (Sibal et al. 2000), also, this process does not require any modification to the existing batch mixing plant.

1.3. INNOVATIVE APPLICATIONS

Innovative applications may be construction method based or design principle based. Some of the relevant issues are discussed in the following. 1.3.1 Construction Method Based

A mixture of aggregate and binding material, at varied proportions, constitute various specifications for road construction, for example, bituminous concrete, built-up spray grout, wet mix macadam, lean cement concrete etc. Discussion on all these standard specifications have been skipped here, rather, some specific mixes and their construction methods are discussed. Emulsified bituminous mix Cold emulsified bituminous mix (EBM) is gaining more and more acceptance for its environmental friendliness and less hazardous construction process. A relative comparison between the EBM and hot bituminous mix (HBM) has been presented in Table 4. It may be noted that though the rate of strength gain in EBM is slower (refer Figure 7), the final strength of EBM is comparable to that of HBM. Table 4. Comparison of hot bituminous mix (HBM) and emulsified bituminous mix (EBM)

Property Heating Setting time Applicability Convenience Energy Uniqueness Economy Safety

HBM EBM Strong heating required, oxidative hardening No heating required, so no occurs oxidative hardening Low High Clear weather with high ambient All weather (wet surfaces, temperatures rainy seasons, cold) Relatively difficult construction than EBM Relatively easy construction Relatively higher requirement Relatively lower requirement Modifiers needed Inherent anti-stripping agents Less costly More costly Hazards from fuming, fire and environmental Free from such hazards pollution

Foamed bituminous mix

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Foamed bituminous mix (FBM) is a foamed mixture of air, water and bitumen. It is produced by injecting very small quantity of water into the hot bitumen, resulting in spontaneous foaming and temporary alteration of the physical properties of the bitumen. Figure-8 represents schematically the manufacture of FBM. Although the foamed bitumen technology was developed more than forty years ago, it is now gaining popularity owing to its good performance, ease of construction and compatibility with a wide range of aggregate types (Transportek 1998). Usage of FBM results in reduction in binder content and transportation costs, as it requires less binder and water than other types of cold mixing methods. FBM can be compacted immediately and can carry traffic almost immediately after compaction is completed ( Jenkins et al., 2003 ). The strength characteristics of FBMs are highly moisture dependent. This is because of the relatively low binder content and high void content of foamed bituminous mixes. FBMs are not as temperature susceptible as HBM. Since larger aggregates are not coated with binder, the friction between the aggregates is maintained at higher temperatures. Foamed bitumen can achieve stiffness comparable to those of cement-treated materials, with the added advantages of flexibility and fatigue resistance (Ramanujam and Kendall, 1999). FBMs usually lack resistance to abrasion and raveling and are not suitable for wearing/friction course applications.

Figure - 8 Schematic presentation of FBM manufacture ( Romanoschi 2003 ) Some specific situations where use of foamed bitumen technology can be considered are:

• A pavement which has been repeatedly patched to the extent that pavement repairs are no longer cost effective. • A weak granular base overlies a reasonably strong subgrade. • Granular base too thin to consider using cementitious binders. • Can be effectively used in desert road stabilization etc. (Jenkins et al., 2003). Relatively high cost, requirement of specific equipment for mix production, sensitivity to aggregate grading and stripping risk are some of the disadvantages with the foamed bituminous mix ( Jenkins et al., 2003 ). Fiber reinforced bituminous mix Addition of various kinds of fibers to the binder and aggregates during mix preparation process results in fiber reinforced bituminous mix (FRBM). Fibers are generally blended with bitumen binder before mixing it with the aggregates to achieve complete coating and even distribution throughout the mix. Research shows that FRBMs develop good resistance to aging, fatigue cracking, moisture damage, bleeding, reflection cracking etc. (Serfass and Samanos, 1996; Maurer et al., 1989). Ultra-thin whitetopping

Overlaying technique of pavement rehabilitation is well known and widely practiced. However, ultra thin whitetopping (UTW) of concrete over existing bituminous pavement is a relatively new concept. UTW can be designed for low-speed, low volume traffic areas such as street intersections, aviation taxiways and runways, bus stops and tollbooths. In this technique, a thin layer of high-strength, fiber-reinforced concrete is placed over a clean, milled surface of distressed bituminous concrete pavement to achieve a full or partial bonding. Bonding makes the two layers behave as a monolithic unit and share the load. Due to bonding, the neutral axis in concrete shifts from the middle of concrete layer towards its bottom. This results in a lowering of stresses at the bottom of concrete layer. Thick composite section behaviour causes the corner stresses to decrease. On the other hand, downward shifting of neutral axis may cause

critical load location to shift from edges to corners thus increasing the corner stresses. Short joint spacing is used to decrease the slab area that can curl or warp thus minimizing the corresponding stresses (MTTP 2004). A schematic diagram of UTW have been presented in Figure-9.

Figure -9 Flexible composite pavement using UTW

UTW is an excellent resurfacing option for deteriorated bituminous pavements which otherwise require frequent repair or overlays. Following are some of the advantages of a UTW system (CAC 2004, Murison 2002):

• It is beneficial for repair of roads and intersections having problems of rutting, cracks, and poor drainage. • It provides improved skid resistance. • Its light colour reflects more light than bituminous pavement. • Its heat-reflecting property can help to lower the average city temperature. • It is less costly to maintain, than conventional flexible pavements, and does not require frequent resurfacing and repairs. • The UTW concrete resists bitumen aging. • The UTW concrete prevents degradation of bituminous surface due to fuel spills. • It causes minimal traffic disruption due to faster construction and repair procedure. • Its small panels are ideal for utility maintenance. Bituminous recycling In recycling method, bitumen and aggregates are separated out (partly or fully) and used again. The specific benefits of recycling of bituminous pavement can be summarized as:

     

Conservation of energy and construction material. Prevention of undesirable rise in height of finished surface and preservation of the existing road geometrics. Reuse of deteriorated road materials which in turn solves the disposal problem. Solution to the problem of scarcity of good quality material. Preservation of the environment. Reduction in susceptibility to reflection cracking.

Bitumen ages due to oxidation with atmospheric oxygen as a result of which resins get converted into asphaltenes (Petersen, 1984). By this process bitumen loses its ductility and becomes more brittle. Recycling is based on the fact that bitumen obtained from old deteriorated bituminous pavement, may still has its residual properties and recycling helps in restoring those residual properties of the bitumen. To judge the suitability for use as a recycled material, aggregates are tested for their gradation and bitumen is tested for its engineering properties. The optimum quantity of reclaimed material to be mixed with fresh material is generally determined from mix design process. Fresh thin (soft grade) bitumen having low viscosity can be used to replenish

the aged bitumen. Rejuvenators (like road oils and flux oils) are sometimes added for improvement in properties of reclaimed bitumen. There are four major technologies exist for bituminous pavement recycling (NCHRP-452). They are (i) hot mix recycling

Here recycled asphalt pavement ( RAP) is combined with fresh aggregate and bituminous binder or recycling agent in a hot mix plant. Mix is transported to paving site, placed, and compacted. (ii) cold in-place recycling

In this the existing pavement is milled up to a depth of 75 to 100mm, RAP, if necessary and recycling agent in emulsion form is introduced, then compacted. (iii) hot in-place recycling

In hot in-place recycling method the existing asphalt surface is heated, scarified to a depth from 20 to 40 mm, scarified material combined with aggregate and/or bituminous binder and/or recycling agent and compacted. New overlay may or may not be provided. (iv) full depth reclamation

Here all the bituminous layers and predetermined thickness of underlying material is pulverized, stabilized with additives, and compacted. A surface course is applied over it. Design Principle Based

This section discusses about the design principle based innovative applications of road materials. Discussion has been divided into two parts viz.,  

Structural design considerations, and Mix design considerations

Optimum pavement design thickness In Mechanistic-Empirical pavement design, generally sustainability of a pavement structure against fatigue and rutting failures is considered, for which the critical responses are: (a) the tensile strain at the bottom fibre of bituminous layer and (b) the vertical strain at the top of the subgrade. A number of design thickness combinations of bituminous and granular layers are possible which satisfies the above mentioned requirement. Standard design charts developed by various organizations (Shell 1978; Austroads 1992; Asphalt Institute 1981; IRC:37-2001) are available; these design charts generally provide thickness composition of bituminous and granular layers, depending upon other input parameters viz. temperature, traffic, design life, subgrade strength, material type etc. A designer can choose any suitable granular layer thickness, and, corresponding thickness of bituminous layer can be read from these charts.

Figure 10. Typical pavement design chart

POINT A - Safe from rutting but oversafe from fatigue considerations. POINT B- Safe from rutting but unsafe from fatigue considerations. POINT C- Safe from fatigue but insafe from rutting considerations. POINT D- Safe from fatigue but oversafe from rutting considerations. POINT E - Unsafe from both rutting and fatigue considerations. Point F- Oversafe from both rutting and fatigue considerations POINT O- Just safe from both rutting and fatigue considerations. Figure 10 illustrates a typical design chart. The design chart consists of two curves: fatigue curve and rutting curve. The fatigue curve shown as COD in Figure 10 represents the points, which are just safe from fatigue consideration. Similarly, the rutting curve shown as AOB in Figure 10 represents those points which are just safe from rutting consideration. Figure 10 shows various points like A, B, C, D, etc. They are safe, oversafe or unsafe from fatigue or rutting considerations. The reader can point the cursor on the respective points to know about their status. In the design chart the fatigue curve and the rutting curve intersects at a point (point O in this case) that may be called as structurally balanced design point (Narasimham, et al., 2001). Thickness of pavement layers chosen according to this point will result in a pavement deign which would fail due to fatigue and rutting simultaneously. Similarly, there could be cost optimal point, where bituminous and granular layer thicknesses are selected such that the total cost of materials used is minimized, without compromising with the structural adequacy of the pavement. The cost optimal point may or may not coincide with the structurally optimal point (Narasimham et al. 2001, Das 2004 ).

Perpetual pavement A perpetual bituminous pavement may be defined as a pavement designed and built to last longer than fifty years without requiring major structural rehabilitation or reconstruction (APA101 2001). This pavement may only require periodic replacement of top wearing surface and recycling of old pavement material (TRL 2001; AA-2 2001). The concept of full depth bituminous pavement is in vogue from 1980s in USA. Nunn and his associates of Transport Research Laboratory, UK found (Nunn et al., 1997) that thick bituminous pavements tend to show long lasting performance and may require only minor surface repairs. California Department of Transportation in collaboration with

University of California, Berkeley (Monismith et al., 2001) first implemented concept of perpetual pavement in a rehabilitation planning project. In full depth bituminous pavement, the thickness is so designed that the fatigue and rutting strains developed are below the permissible limit (MS-1 1999 ). If the thickness is chosen to be sufficiently large so that the fatigue strain is close to the endurance limit, then the fatigue life becomes very long, and the pavement may be said to have attended 'perpetual life'. A perpetual pavement, in general, is made up of the following layers:   

The top wearing surface is designed in such a way that it is water-tight as well as removable and hence replaceable. Stone Matrix Asphalt (SMA) or Open Graded Friction Course (OGFC) are recommended. They also produce less noise due to tyre-pavement interaction. The intermediate layer is constituted with good quality aggregates and designed to be strongly resistive to rutting. The bottom part is made resistant to fatigue cracking by making it rich in bitumen and choosing a gradation that has less voids.

Figure 11 Layer composition of a perpetual pavement. Figure-11 schematically represents the layer composition of a typical perpetual pavement.

A perpetual pavement is a full depth bituminous pavement in most of the cases. The principles based on which it is designed (mix design and structural thickness design) are the following:   



The pavement layers are chosen in such a way that they are rut resistive. The pavement is chosen to be adequately thick such that the vertical subgrade strain is low. Since subgrade contributes to the major part of rutting, low vertical subgrade strain would cause low level of rutting. The wearing surface should be adequately water-proof. The surface should be so designed that it can be repaired or recycled and the whole pavement will not require any major reconstruction (AA-2 2001). The thickness of the bituminous layer is chosen in such a way that the horizontal tensile strain (ε t) developed is less than the endurance limit (refer Figure-12) of the bituminous mix, hence its laboratory fatigue life (N) becomes infinity (AA2-2001, Nunn et al. 1997). It is justifiable to design the pavement as 'bottom rich' (refer to next section), which shifts the endurance limit to higher level. The temperature gradient tends to be steeper towards the surface of the pavement (TRL 2001, Newcomb 2001) as shown schematically in Figure-12. Therefore the bituminous mixes with temperature susceptible binder should be avoided as surface course. Use of modified binder could be helpful in this regard.

Figure 12

Idealized diagram of fatigue characteristics of bituminous mixes.

Rich bottom bituminous pavement

Increased binder content above the optimum content can appreciably enhance the fatigue life. Higher bitumen content increases the thickness of the binder film between aggregates resulting in lower stress in the binder film, and thus the fatigue life is improved (Sousa et al., 1998; Harvey et al., 1996). However, with increased amount of binder content, the bituminous mix tends to be softer and thereby its stiffness modulus value may fall. A mix designer's objective would be to achieve both high stiffness and high fatigue life. This mutually contradictory situation can be handled by using a bituminous pavement layer where it is made richer in binder content towards the bottom layer(s). Since fatigue cracks start from bottom of bituminous layer, higher bitumen content helps to give greater restraint against fatigue cracking. This concept has been termed as 'rich bottom pavement' (Monismith et al., 2001; Harvey et al., 1997; Harvey and Tsai 1996). Figures 13 and 14 provide two such options of achieving this condition. In Figure-13, quantity of bitumen is used more towards the bottom of the layer. In Figure-14, two different bituminous mixes are used in two layers. Out of three possible alternatives, alternative-II turns out to be the best alternative.

Figure 13 Rich bottom pavement

Figure 14 Two grades of bitumen used in two layers Inverted pavements

   

Inverted pavement system, or inverted base, is a high depth pavement whose supporting layers are thicker and stiffer than top layers. The system consists of a thin bituminous concrete (BC) layer provided on top of a graded aggregate base (GAB) layer. A portland cement-treated stiff base layer is provided at the bottom. This arrangement causes the critical stress/strain plane to be located at the interface of the BC and GAB layers. Thus only the top portion of the inverted pavement structure absorbs the traffic loads as compared to conventional design where thick sections are required for load distribution. Research by South African Roads Board (SARB 2004) and Georgia Department of Transportation, has shown that an inverted base provides enough structural performance to support traffic loadings up to 100 million Equivalent Single-Axle Loads (ESAL s) with a maximum two inch bitumen riding course (Halsted, 2002). According to SARB, this type of system proves to be more cost effective for construction of long lasting pavements.

Bituminous pavement with cemented base

   

The cemented bases are derived from aggregates mixed with some binding material. Since it is bounded layer, it also has some fatigue life. Thus, unlike the unbound granular base, the cemented base layer contributes to some fatigue life, which may give rise to comparative reduction of design thickness of bituminous layer (Das and Pandey 1998). The stiffness modulus of cemented layer is generally found to be much higher than granular sub-base; however, due to shrinkage cracks, the stiffness modulus falls rapidly. This change in stiffness values at different stages of the design life has been schematically shown in Figure15(a) and Figure-15(b) presents a typical design chart for design of bituminous pavement with cemented base made up of lime-soil mixture.

15 (a) Change of elastic modulus of cemented bases at different phases.

15 (b) A typical design chart of bituminous pavement with cemented base (LS º lime soil)

Mix design considerations Non-standard gradation

The fatigue life of the mix can be increased by increasing the bitumen content.

But, Voids in Mineral Aggregates (VMA), being fixed for a given gradation and compaction level, increase in bitumen content will result in less Air Voids (VA), which is undesirable for a mix.

However one can deviate from the specified gradation in order to come up with a new gradation, which possibly can give rise to better fatigue performance, yet without compromising with the VMA and Marshall-stability requirements. Stone matrix asphalt

Stone matrix asphalt (SMA) is a gap-graded bituminous mix with high percentage of coarse aggregates with high bitumen content. Gap gradation aims at maximizing stone-to-stone contact. This gives a structurally strong mix due to efficient load distribution through the stone-matrix skeleton. The drawback of this method is the absence of medium sized aggregates due to gap gradation. This may arise possibility of drain-down of low-grade penetration bitumen through the stone matrix . To check this possibility, modifiers, such as cellulose fibers, are used to hold the bitumen in place (Better Roads 2003; GDOT 1995; Decoene et al., 1990). Porous pavement

     

Porous pavement is a special type of pavement which allows surface water to pass through it, thereby keeping the road surface water-free, as well as providing drainage outlet to storm water. Porous pavement may be effectively used in light traffic areas like parking areas, airport taxiway and runway shoulders, footpaths, playgrounds etc. provided that the subsoil drainage, groundwater level and topography of the area is suitable (Michele, 2003; USEPA 1999; DEQ 1992). Pavement structure consists of a top porous bituminous layer placed over a filter layer below which a highly permeable open-graded stone layer (known as reservoir course). A geotextile layer is placed at the bottom to screen off fine soil particles. Porous bituminous layer consists of gap-graded aggregates (lower percentage fines), held together by a fiber-bitumen blend, giving a matrix structure which allows movement of water through its fine voids. Besides load bearing, the reservoir course stores the runoff water (in the void spaces in aggregate layers) until it can infiltrate into the soil beneath. Porous pavement has been found (RPL 2001) to be quite effective in reducing noise levels, splash and spray during rains, and aquaplaning tendency thereby improving the wet skid resistance.

CLOSING REMARKS

Certain standard methods are followed for road design and construction. They are modified from time-to-time to match with the technological advancements. Certain modifications in the mix design or structural design can give rise to substantial economy in terms of the longevity of the pavement or the cost of the material concerned. Recapitulations

Cement is manufactured by heating a mixture of limestone, iron ore, gypsum, clay and other ingredients. Cement concrete is a mixture of coarse aggregates, fine aggregates, cement and water, in suitable proportions. Through mix design, suitable proportions of the ingredients of concrete are estimated considering strength, workability, durability and economics. Workability test and air-content test are the tests generally conducted on fresh concrete. Compressive strength, tensile strength, modulus of rupture, elastic modulus, Poisson's ratio, creep and shrinkage, durability, thermal expansion coefficient etc are the tests conducted on hardened concrete. Various modifications and inovatory applications of pavement materials and pavement design brings in better performance and economy.

Analysis of Concrete Pavement Objectives

The objective of this lecture is to review the basic concepts involved in the analysis of concrete pavement structure. It presents a general formulation for load and temperature stresses in concrete pavement. Solutions obtained for estimation of pavement stresses are also mentioned. Introduction

The concrete pavement is subjected two types of stresses, load stress and temperature stress. The load due to traffic acts vertically downward, and the stress is, therefore, tensile at the bottom of the pavement slab. Temperature stress is generated due to temperature gradient across the depth of the slab.

Analysis of load stress

A concrete pavement is generally idealized as slab (or, plate) resting on elastic foundation. So as to develop the formulation for load stress calculation, the theory of plates is to be recapitulated first. Basic theory of plates

For analysis purpose, a plate can be assumed as infinite in both the directions, semi-infinite, finite in one direction, or, finite in both the directions and accordingly the reference coordinates can be chosen. Figure 16 explains this configurations schematically. For a basic plate analysis, it is assumed that the plate is homogenous, isotropic and elastic material and the cross section normal to the neutral axis remains plane before and after bending and, thus, there is no deformation along the thickness of plate. Therefore, the bending stress, becomes a plane stress case. For small deflection (

at any point across the depth ( ) and the analysis ) of plate (Timoshenko, and Kreiger 1959), it can be written,

where, represents the deflection in Cartesian coordinate system along the z-direction (thickness) and is the radius of curvature in the respective direction. The negative sign represents that the upward curvature is due to downward deflection.

Figure 16: Various possible dimensions of a plate

Plate bending theory can be grossly grouped into two, thin plate theory and thick plate theory. For concrete pavement analysis purpose, it is generally assumed as thin plate. The assumption of thin plate (called as Kirchhoff plate) bending theory is that the thickness of the plate ( ) is small as compared to the other dimensions. Thus, the effects of

and

on bending are neglected where,

represent the bending stress due to external load in

respective planes and

represent the depth from the neutral axis (Timoshenko, and Kreiger 1959). Thus,

where, is the strain in the respective direction due to external load. So, the strain-deflection relationship due to bending may be presented as:

The strain-stress relationship due to bending may be represented as:

where, strain

and

are the Young's modulus and shear modulus respectively (i.e. relationship due to external

). The bending stressload becomes:

Therefore, the stress-deflection relationship may be expressed as:

(4)

(5)

(6)

If , are the bending moments per unit length due to load, parallel to and axis respectively, and is the respective twisting moment in the plane, then (Timoshenko, and Kreiger 1959),

(7)

(8)

(9)

where, flexural rigidity,

Let,

and

. Also, the stresses may be expressed as:

are the shear forces per unit length parallel to

and axis, then

(10)

(11)

If is the net pressure (downward positive) over the plate surface which may include the external load, subgrade reaction as well as self weight of the plate, then from the equilibrium considerations:

(12)

Therefore, differentiating equation (10) with respect to and equation (11) with respect to and, then, substituting in equation (12), the equilibrium equation becomes (Timoshenko, and Kreiger 1959):

(13)

where,

(Laplace biharmonic operator).

When the curvatures in x and y directions are equal such as the case of symmetric interior loading, the deflection ) with free edges may be expressed as shown below:

(

Equilibrium equations based on foundation types

The equilibrium equation(13) takes different forms depending on the loading condition as well as the foundation n alltypes. In the following three different forms of the equilibrium equations are derived for three different types of foundations (subgrade). These three foundation models are (i) Winkler foundation (ii) Pasternak foundation and (iii) Kerr foundation. I these three cases, external pressure, q is applied over the surface of the slab. In case of concentrated load, the equilibrium equations are valid, except at the point of load application.   

Slab resting on Winkler foundation Slab resting on Pasternak foundation Slab resting on Kerr foundation

Slab resting on Winkler foundation

In this type of foundation, the subgrade is assumed to be composed of closely spaced linear springs and subgrade shear strength is neglected. The subgrade reaction is proportional to the deflection (equivalent to dense liquid foundation) and the proportionality (spring) constant is called as modulus of subgrade reaction (k). So, the upward subgrade pressure ( ) is . The equilibrium equation of plate resting on Winkler foundation becomes:

(14)

where, radius of relative stiffness,

.

The closed form solution for deflection due to slab wight (unit wight= ) for finite dimensions ( ) supported over an area in the middle by subgrade reaction may expressed as (Timoshenko, and Kreiger 1959):

where,

.

Slab resting on Pasternak foundation

In this type of foundation, the shear interaction between the Winkler spring element is incorporated and subgrade shear force is assumed to be proportional to the variation in deflection in the soil layer (Cauwelaert et al. 2002). Therefore,

where, and are the subgrade shear forces per unit length acting on the plane orthogonal to and direction respectively and is called as subgrade shear modulus. So, the net upward vertical pressure ( ) is:

Therefore, the equilibrium equation for plate resting on Pasternak foundation become:

(15)

The Winkler foundation is a special case of Pasternak foundation (i.e when,

).

Slab resting on Kerr foundation

In Kerr foundation, the subgrade is characterized with two spring constants ( ) along with presence of subgrade shear interaction. The relationship between the upward subgrade reaction ( ) and deflection ( ) of the slab as follows (Cauwelaert et al. 2002):

Therefore, the equilibrium equation (13) for Pasternak foundation becomes,

(16)

The Pasternak foundation is a special case of Kerr foundation (i.e when,

).

Temperature stress analysis

Most of the studies show that the temperature distribution in concrete pavement is nonlinear. Let, represent the pavement temperature at depth from the mid-surface (positive downward) and is the reference temperature at which the slab is free from any temperature stress. It is also assumed that elastic modulus ( ) and Poisson's ratio ( ) does not change with temperature and also the pavement temperature distribution of the pavement, with respect to time, is fixed. So, if the slab is fully restrained, the restrained strain ( temperature change from

to

) due to

will be:

(17)

The corresponding stress (

), may be expressed as:

(18)

(19)

where, is coefficient of thermal expansion of concrete. The positive strain indicates the compressive stress (negative). It may be noted that the shape of the stress diagram is similar to the temperature profile. The above stresses (equation 18 and 19) can be divided into three components, (i) axial, (ii) bending and (iii) residual. This has been explained schematically in Fig. 17 and Fig. 18, for day time and night time conditions respectively (Choubane and Tia 1992). The various stress components are discussed further in the following.

Figure 17. Various components of stress during day time comdition

Figure 18: Various components of stress during night time condition

  

Axial stress component Bending stress component Residual stress component

Axial stress component

It is assumed that the axial stress ( axial temperature component,

), which is constant through the thickness of the slab, is generated due to

. The axial stress,

the total thermal force produced by

, can be obtained by equating thermal force due to

as shown in the following (Ioannides and Khazanovich 1998):

to

If

are axial strain and axial stress respectively, then

Most of the time concrete pavement is allowed to expand or contract through various joints. Thus, in such a situation, =

=

.

Bending stress component

In a similar way, it is assumed that the bending stress ( ), which varies linearly through the thickness of the slab and assumes a value of zero at the mid-plane of the cross-section, is generated due to bending (linear) temperature component,

. The bending stress,

force produced by

, can be obtained by equating thermal force due to

to the total thermal

as shown in the following (Ioannides and Khazanovich 1998):

Accordingly, bending strain (

) and bending stress (

) may be expressed as given below:

The maximum values of strain, stress and moment occur at the top ( these values can be calculated as:

) and bottom (

) and

Most of the time the concrete pavement is restrained against bending. This restraint is primarily due to self weight of the slab. Thus, the strain mentioned above is the restrained strain, and the bending stress does develop in concrete pavement. Residual stress component

The shape of is arbitrary. Thus, combination of and does not add up to . Thus, is the residual temperature component, obtained after deducting both the axial and linear temperature components from total temperature. The value of can be calculated as follows:

Thus, the restrained strain and the corresponding stress can be obtained as:

Solutions for load and temperature stress

The formulation for the load stress and temperature stress are solved by various researchers for various boundary conditions. Some such solutions are presented in the following: 



Solutions for load stress o Interior stress due to loading at interior (Westergaard 1926) o Edge stress due to loading at edge (Westergaard 1926 o Corner stress due to loading at corner (Bradbury 1938, Fwa et al. 1996, Westergaard 1926) Solutions for temperature stress

Solutions for load stress

The following are various solutions for load stress obtained by various researchers. 

Interior stress due to loading at interior (Westergaard 1926)

 

Edge stress due to loading at edge (Westergaard 1926 Corner stress due to loading at corner (Bradbury 1938, Fwa et al. 1996, Westergaard 1926)

Interior stress due to loading at interior (Westergaard 1926)

Edge stress due to loading at edge (Westergaard 1926)

Corner stress due to loading at corner (Bradbury 1938, Fwa et al. 1996, Westergaard 1926)

where, radius of resisting section, if and Euler's constant,

; if

or

;

.

Solutions for temperature stress

The Westergaard's (1926) equation for maximum tensile stress at the top for cental area of infinite slab due to negative temperature differential ( ) is:

(17)

The Westergaard expression for deflection and maximum tensile stress of semi infinite slab ( and )

(18)

(19)

where,

and

The maximum occurs at (Edge

as expressed above. and the tensile stress at the edge location (

),

; condition)

For the slab with finite width ( stress equations are -

) and infinite length (

), the deflection and

(20)

(21)

where width (

. Similar solution can be obtained for the slab with finite length ( ) and taking,

) and infinite

.

The Bradbury (1938) equation for maximum tensile stress at the edge ( ) and interior ( temperature gradient and finite slab with all edges free, over Winkler foundation are:

) location with linear

(22)

(23)

where, ;

and,

and

are same as above.

The downward vertical displacement due to weight of slab over a dense liquid foundation may be represented as:

The Westerdard solution is valid when (slab in full contact). For large negative temperature differential ( ) the slab may curled up and considering the gap due to curling, a closed form solution for semi-infinite slab can be presented as (Tang et al. 1993):

(24)

where,

and

.

The maximum occurs at . When , then, and thus it becomes the Westergaard's solution. For slab with finite width and infinite length, Westergaard's solution is valid when and

. The general solution proposed by Tang et al. (1993)is:

(25 )

Recapitulation

   

Concrete pavement is generally modeled as thin plate resting on elastic foundation. The load stress is tensile at bottom. The solutions are different for different types of foundations, such as Winkler, Pasternak, Kerr etc. The temperature stress develops in concrete pavement due to the change of temperature and the existence of temperature gradient across its depth. Most of the studies show that this temperature distribution is non-linear. The temperature stress can be considered to be composed of three stress components, viz. axial, bending and residual. Generally, the axial stress in concrete pavement gets dissipated due to provision of various joints. Primarily self-weight provides the restraint against bending (due to temperature gradient) of concrete pavement. During the day time, a concrete pavement is expected to experience tensile stress below the neutral axis.

Various Design Approaches Objective 

The objective is this lecture is to introduce the basic principle of concrete pavement design, and discuss briefly the provisions prescribed in various design guidelines.

Introduction

The concrete pavement can be of three types, jointed plain concrete pavement (JPCP), jointed reinforced concrete pavement (JRCP) and continuously reinforced concrete pavement (CRCP). There could be another variety - prestressed concrete pavement, which has specialized application, such as airport runway pavement. The relative advantages and disadvantages are mentioned in Table-5 (Swanlund and Vanikar 2002).

Table -5 Advantages and disadvantages of various types of concrete pavements (Swanlund and Vanikar 2002) Pavement type

Jointed plain concrete pavement (JPCP)

Advantages

Disadvantages

Reliable design and can be Joint maintenance is costly used at all locations. and may impair future performance after rehabilitation. Jointed reinforced concrete Fewer joints than JPCP. - dopavement (JRCP) Damage due to pavement cracking is less severe than JPCP, because the reinforcement holds the cracked portion in position. Continuously reinforced No joints, hence smooth High initial cost, complex to concrete ride and long service life. construct. Maintenance is pavement (CRCP) costly and difficult.

JPCP is the most popularly used pavement. The present lecture discusses various design approaches for design of JPCP pavements. The design approaches for JRCP and CRCP have been discussed elsewhere . Design parameters

The design parameters can be primarily divided into three categories, material, traffic and environmental parameters and are discussed in the following. Material parameters

The elastic modulus, Poisson's ratio, fatigue life, modulus of rupture etc. are the engineering parameters used for the structural design of the concrete pavement. The input parameters are either found out experimentally or estimated from various recommendations provided in the design guidelines (ACI 2001, ASTM 2003, IS-456). Statistically suitable design value is to be adopted if there are variations in the input parameters (Chakroborty and Das 2003). The modulus of subgrade reaction (k) is generally used for characterization of subgrade strength. k in idealized model represents the spring constant of a dense liquid foundation. The k value is obtained by performing plate load test on the subgrade. The IRC:58 (2002) provides a table with suggested k values when CBR values of subgrade are known. Concrete slab, in general, are constructed on bound, stabilized, or unbound sub-base layer, and not directly on subgrade. The sub-base layer provided below the concrete pavement serves certain purposes (Austroads 2004), such as, 1. It provides uniform support to the concrete slab. 2. It limits pumping at joints and slab edges. 3. It controls shrinkage of concrete slab or swelling of subgrade soil The effective k value that includes the sub-base layer and the subgrade should be used for design of thickness of the concrete slab. This effective k value can be obtained by performing plate load test on the constructed sub-base, or by computational means. Various guidelines (IRC:58 2002, AASHTO 1993, PCA 1984, Austroads 2004) suggest suitable values of effective k when the type and thickness of the sub-base layer is known. Traffic parameters

The concept of axle load, wheel configuration, traffic volume, lane traffic, traffic growth rate, and calculation of axle load repetitions have already been introduced in the other web course on Transportation Engineering-I. Interestingly, vehicle damage factor (VDF) (also known as Truck Factor (MS-1 1999)) is not considered in concrete pavement design. This is due to the fact that typically for concrete pavement design considers the individual damages created by individual axle load groups are considered. To take care of the variability of traffic axle load measurement, and unpredicted heavy truck load, a factor called the load safety factor (LSF) is generally used in concrete pavement design and is multiplied with the total equivalent standard axle load repetitions (Austroads 2004, IRC:58 2002). Environmental parameters

The temperature and the subgrade moisture are the environmental parameters that affect the concrete pavement design. The temperature variation and the temperature differential induces temperature stresses to the concrete pavement. Temperature issues in concrete pavement has been discussed in the lecture 'analysis of concrete pavement' . The subgrade moisture level affects the subgrade strength and also it induces curling stresses due to moisture gradient. Design life

Design life is the number of years (or number of standard axle repetitions) for which the pavement is being designed. A pavement is expected to serve satisfactorily within the design life. For concrete pavements 20 to 40 years may be assumed as the design life (PCA 1984, IRC:58 2002). Basic design principle

Though different approaches for concrete pavement design are suggested in various guidelines, the design principles tend to remain similar across different guidelines, for example, PCA (1994), Austroads (2004), NCHRP (2004), IRC:58 (2002) etc., except the AASHTO (1993) provisions, which is based on empirical approach. The basic steps involved in the design of concrete pavement method can be summarized as follows:

 





The developed stresses due to load for a trial thickness of the concrete slab are calculated for various loading configuration and the critical one is chosen. The axle loads are generally divided into different axle load groups and the load stresses are calculated individually. The ratio between the load stress and Modulus of Rupture (MOR) is known as stress ratio . The stress ratio determines how many repetitions the pavement can sustain (i.e. allowable traffic repetitions ) for the individual axle load group. If the stress ratio is 0.55 or lower, it can withstand virtually infinite number of traffic repetitions (PCA 1984). The ratio between the allowable repetitions to the expected traffic repetitions is the damage fraction. The calculation process is repeated for various axle loads (sometimes, for various seasons, or various timings of the day), and the sum of individual damage fractions ( cumulative fatigue damage ) should be less than equal to one for pavement design being safe. If found unsafe, the trial thickness is changed and the design process is repeated. The design process may also include considerations for temperature stress, moisture stresses, and erosion distress.

Various design approaches

A brief discussion on concrete pavement design approaches suggested by various design practices, viz. PCA method (1984), Austroads method (2004), AASHTO method (1993), NCHRP mechanistic-empirical method (2004) and the Indian Roads Congress (IRC) method (2002) are placed in the following: Portland Cement Association (PCA) Method

The PCA method is based on Westergaard, Picket and Ray's work and further theoretical analysis by Finite Element Method (Huang 1993). The data used to develop the PCA method is generated from various road tests, like, ASSHO road test, Arlington test (conducted by PCA), Bates test road, and Maryland road test (PCA 1984). The PCA design method is based on the following two considerations (PCA 1984):   

The fatigue damage on the concrete slab, due to repetitive application of traffic load is estimated. The cumulative fatigue damage principle is used to estimate the design thickness of the slab. Edge stress between the mid-way of the transverse joint is taken as critical configuration. The stresses due to warping and curling (due to temperature and moisture gradient) are not considered in the fatigue analysis as per PCA recommendation, because, most of the time the stresses generated are subtractive to the load stress. The possibility of erosion of pavement materials placed below the concrete slab is evaluated. The rate at which the slab is deflected due to axle load is used as a criterion for erosion. Theoretically, it can be shown that a thin pavement with smaller deflection basin is subjected to faster rate of deflection, compared to thicker slab. Hence thin slab is more susceptible to erosion. In a similar way the cumulative erosion damage is calculated for individual axle load groups. If this value is greater than one, then the design needs to be revised.

Austroads method

Austroads method has been adopted from PCA (1984) approach with modifications suited to Australian conditions (Austroads 2004). A bound mix or lean cement concrete is used as sub-base material. For a given CBR value of the subgrade and given thickness of cemented sub-base, the effective subgrade strength can be obtained from the chart provided. Figure-19 schematically shows such a chart. For different levels of traffic, Austroads (2004) suggests minimum values of the base thicknesses to be provided.

Figure-19 Schematic diagram of Austroads (2004) chart for estimation of effective subgrade strength

As per Astroads (2004), the concrete pavement slab thickness, for a given expected traffic repetitions, is designed considering the (i) flexural fatigue of the concrete slab and the (ii) subgrade erosion arising out of repeated deflections. For both the considerations, equations are suggested to calculate the allowable traffic repetitions. If the allowable traffic is less than the expected traffic, the design is revised by increasing the slab thickness. The concrete shoulders adopted are of integral type or structural type . AASHTO method The AASHTO method (1993) for design of concrete pavement has evolved from the AASHO road test (AASHO 1962). The AASHTO pavement design follows an empirical approach. Pavement performance in terms of present serviceability index ( PSI ), loss of serviceability, subgrade and sub-base strength, cumulative traffic, properties of concrete, joint load transfer efficiency, drainage condition, overall standard deviation and reliability are the input parameters considered in the pavement design. The PSI value of the fresh pavement is assumed as 4.5 and the pavement is deemed to have failed when the PSI value reaches 2.5. The resilient moduli of the subgrade and sub-base materials are determined in the laboratory simulating the seasonal moisture content and stress situation. Suggested values are also available for given moisture content, plasticity index etc. The composite modulus of subgrade reaction (k) is estimated from modulus of subgrade reaction and elastic modulus of sub-base for various seasons and the depth of rigid foundation and the thickness of the sub-base. Design equation as well nomographs are available to estimate the slab thickness (D) from these input parameters. NCHRP mechanistic-empirical (M-E) method

The NCHRP (2004) has recently developed concrete pavement design procedure based on mechanistic-empirical (ME) approach. This approach attempts to reduce the extent of empiricism prevalent in the existing AASHTO (1993) guidelines. This proposed NCHRP pavement design system is modular in nature, that is, the design approach can be modified by parts (as and when new knowledge is available) without disrupting the overall design procedure. This approach also can take care of various wheel-axle load configuration (Khanum et al. 2004). As per this approach trial thickness of the slab is first assumed, and the stress, strain and displacement values are obtained. From these values, the performance of the pavement in terms of distresses (such as faulting, cracking) and smoothness and predicted. If these predicted performance parameters does not satisfy the required performance for a given reliability, the design revised. The design approach includes a large data-base as input parameters, for example, average daily traffic, traffic growth rate, traffic composition, hourly weather data on air temperature, precipitation, wind speed, percentage sunshine, relative humidity, pavement material engineering parameters, ground water depth, infiltration, drainage, hydraulic conductivity, thermal conductivity, heat capacity etc. The temperature stress is considered in this method, but the temperature profile is linearized to enhance computational efficiency (NCHRP 2004). Indian Roads Congress (IRC) method

The Indian Roads Congress (IRC) guidelines, IRC:58 (2002), has adopted the Westergaard's equation to estimate load stress and Brdabury's equation to estimate temperature stress. The load stress is highest at the corner of the slab, lesser in edge and least in the interior. The order of variation of temperature stress is just the reverse of this. As per IRC:58 (2002), it is recommended that the design needs to be done for edge stress condition and subsequently check needs to be performed for corner stress condition so as to finalize the design. The following are the steps followed as per IRC:58 (2002) guideline for the design of concrete pavement:     

The input parameters are obtained to formulate the design problem. The joint spacing and the slab dimensions are decided. If there is a bound sub-base layer over the subgrade, a suitable value of effective k is to be adopted. A trial thickness of the concrete slab is assumed. The edge stress is estimated for various axle loads from the given charts. Figure-20 schematically shows such a chart. The cumulative fatigue damage principle for fatigue is applied to check the adequacy of the slab thickness. The sum of edge stress due to load for the highest axle load group and the temperature stress should be less than the MOR of concrete, otherwise the design is revised. The adequacy of corner stress is checked with respect to MOR value and accordingly the design is finalized. Westergaard's corner stress formula is for estimation of corner stress due to load, and the corner stress due to temperature is assumed to be zero.

Figure-20 Schematic diagram of IRC:58 chart for estimation of load stress at the edge (IRC:58 2002) Recapitulation



 

The fatigue and erosion damages are generally considered as primary structural factors which govern the concrete pavement design by mechanistic-empirical (M-E) approach. Cumulative damage principle is employed to evaluate the expected level of damage. Faulting and smoothness are the other considerations in the design. The design guidelines such as PCA (1994), Austroads (2004), NCHRP (2004), IRC:58 (2002) are based on ME approach, whereas, the AASHTO (1993) approach of pavement design is purely empirical. While considering the cumulative fatigue damage (in M-E approach), the load stresses are primarily considered. The stresses due to temperature and/or moisture gradient are ignored in PCA (1994) and Austroads (2002) guidelines, but are considered in IRC:58 (2002) and NCHRP (2004) guidelines.

Design of Dowel Bars Objective

The objective of this lecture is to introduce the concept of design of dowel bars. Introduction

Dowel bars are placed at the transverse joints of concrete pavement and they take part in partial wheel load transfer from one slab to its adjacent slab. The dowel bars also allow axial thermal expansion and contraction of the concrete slab along the axis of the dowel. The bars are generally made up of mild-steel-round-bars of short length. Around half of the length of this bar is embedded in one of the concrete slabs and the remaining portion is bonded in the other adjacent slab. One end of the bar is kept free for the movement during expansion and contraction of the slab due to change in temperature.

The performance of the dowel bar system drastically falls when voids are created between the dowel bars and the concrete slab (Porter 2001). Such a situation develops stress concentration and failure may take place rapidly. The void space may get filled with water and corrosion action to the dowel bars may get aggravated. Subsequently, the load transfer mechanism tends to fail and the differential settlement of the concrete slab may occur (Porter 2001). Differential settlement may further cause breakage of slabs due to impact loading of the vehicles. Thus, the design adequacy of dowel bar system and proper placement to the concrete pavement slab is an important consideration. Analysis of dowel bar Analysis

Recall the formulation developed as Equation 13 , in the lecture on 'analysis of concrete pavement' , representing beam on elastic foundation. By putting appropriate boundary conditions to that equation, for a semi-infinite beam with a moment, Mo, and a point load, P, the following solution emerges for Winker's foundation (Porter 2001, Timoshenko and Lessels 1925 ):

(25)

where,

y = the deflection along the x direction, , and is called as the relative stiffness of the beam on the elastic foundation, k = spring constant, E = elastic modulus of the beam and I = moment of inertia of the beam section. From Equation 1, the deflection of the dowel bar at the face of the joint, yo, can be obtained as (Friberg 1940):

(26)

where, relative stiffness of the dowel bar resting on concrete (assumed as elastic foundation), k d = modulus of dowel support, bd = dowel bar width (i.e. diameter), Ed = elastic modulus of the dowel bar, Id = moment of inertia of the dowel bar, Pd = load transferred through the dowel bar and z =joint width. Though the above equation assumes dowel bar to be semi-infinite, Porter (2001) showed that Equation (26) gives a reasonably good estimate of deflection obtained from more rigorous analysis. Thus, the bearing stress developed, σbd, can be expressed as the product of modulus of dowel support ( kd) and the deflection at the face of joint ( yo) (Porter 2001), i.e. (27) For a successful dowel bar design, the value of σbd needs to be kept lower than the allowable bearing stress of concrete, σbd, specified as (ACI 1956, Porter 2001, IRC:58 2002):

(28)

where, σbd is expressed in MPa, fck = characteristic compressive strength of concrete in MPa and bd is the diameter (i.e. width) of the dowel bar in mm, Joint load transfer efficiency

Ideally, the dowel bar system should be able to transfer the whole load applied to it. However, voids developed due to repetitive loading reduce the joint load transfer efficiency ( JLTE ) of the dowel bar. The JLTE (expressed in percentage) of a dowel bar can be defined as (Porter 2001, Ioannides and Korovesis 1992 )

where, Pa is the load applied to the dowel bar. The force transmitted being somewhat difficult to measure, joint load transfer efficiency is sometimes measured in terms ratio of deflections. If impact loading (generally, falling weight deflectometer ( FWD ) is used for this purpose) is applied near the transverse dowelled joint of a slab, the JLTE can be defined as the percentage of the deflection of the unloaded slab with reference to the deflection of the loaded slab. Figure 1 schematically shows two extreme situations as JLTE = 0% and JLTE = 100%. Various researches and documents (Porter 2001, Yoder and Witczak 1975, ACPA 1991 ) suggest that the load transfer efficiency varies between 35 to 50%.

Figue-21 Schematic diagram explaining the deflection based load transfer efficiency (Chakroborty and Das 2003) Distribution of load

A part of the load applied is shared by the dowel bar system. Essentially, this load is not shared by only one dowel bar, rather, it is shared by a group of dowel bars. (These dowel bars are placed at some designed interval). Thus, it is important to know (i) how many dowel bars participate in load transfer, and (ii) how is this load shared across the various participating dowel bars. Fridberg (1940) suggested that a length of up to 1.8 × radius of relative stiffness (refer to Equation (11) of the lecture 'analysis of concrete pavement' for definition) participate in the load transfer. It is also suggested that the load may be taken as linearly varying with maximum share taken by the dowel bar which is just vertically below the wheel. Tabatabaie et al. (Porter 2001) suggested that instead of taking the factor as 1.8 it should be taken as 1.0 . From design point of view, the wheel can be placed in two ways over the transverse edge of the slab, viz. case (i) the wheel at one edge or, case (ii) the wheel is at the middle. Obviously, the maximum load shared by a dowel bar in case (i) will be more than case (ii) . Hence case (i) would govern the design of dowel bar.

Design of dowel bar

By the term design of dowel bar, it is meant to estimate the dowel bar diameter and spacing. One of them can be assumed as known or fixed, and the other is estimated. Dowel bars are generally designed from two considerations: (i) Bearing stress approach, where the developed bearing stress is set equal to or less than the allowable bearing stress, and (ii) Relative deflection approach, where the relative deflection of the joints is not allowed to exceed some maximum specified value. Example design An example on design of dowel bar system from bearing stress consideration is presented in the following. Problem

A design wheel load of 65kN is applied on to the concrete pavement slab, and 50% of the load is assumed to be transmitted through the dowel bar system. Assume the characteristic compressive strength (fck) of concrete (used in the concrete pavement) is 40MPa, the transverse joint width ( z ) is 15 mm, radius of relative stiffness (l) of the concrete slab as 950mm, Elastic modulus of steel (E) as 2.0 x 105 N/mm2 and modulus of dowel support (kd) as 415N/mm 3 . Design the dowel bar system. Solution Assume, 32mm diameter dowel bars are used. The allowable bearing stress (σba ) can be obtained from Equation (28) as:

= 29.23 MPa

Assume, dowel bar spacing as 300 mm center-to-center. Thus, in 950 mm of effective length (using Tabatabaie et al. criterion) , 4 dowel bars can be placed.

Assuming, the load carried by the dowel bar which is just below the wheel, is Pd, one can write the load distribution equation of the dowel bar system as:

or, Pd=15437.5 N Now,

0.0238 per mm From Equation (27), the developed bearing stress in dowel bar

=

N/mm2 = 27.20 MPa

It is noted that center.

<

, thus the design is safe. Thus, dowel bar of 32 mm may be provided with 300mm center-to-

Closing remarks

The side of the dowel bar does not take any shear force, thus it is suggested that a dowel bar with elliptical crosssection will provide more effective area for distribution of stress (Porter 2001). This has been schematically explained in Figure 22.

Figure 22. Elliptical cross-section has more effective area for shear stress distribution (Porter 2001)

While designing the thickness of the concrete pavement, generally the stresses at edge, corner and interior are considered. However, the stresses at the edge of the transverse joint also need to checked, considering the situation when dowel bars fails to perform load transfer ( Porter and Guinn 2002 ). Corrosion of the dowel bars, usage of fiber reinforced polymer dowel bars, estimation of modulus of dowel reaction, dowel-slab interaction and their fatigue (Porter and Guinn 2002) are some of the emerging issues which demand further research and understanding. Recapitulation



Dowel bars are placed at the transverse joints of concrete pavement and they take part in load transfer from one slab to the adjacent slab. The dowel bars also allow axial thermal expansion and contraction of the concrete slab along the axis of the dowel.

 

Only a part of the wheel load is transmitted through the 'participating' dowel bars. The dowel bar which is just vertically below the wheel carries the maximum load. Loads carried by the other participating dowel bars decrease proportionately as these move away from the point of load application. The dowel bar system is designed from the bearing stress and deflection criteria.

Design of Tie Bars Objective

Objective of this lecture is to introduce the design concepts of tie bar. Introduction

Tie bars are used across the longitudinal joint of concrete pavement. The purpose of tie bar is to hold the concrete slabs together (refer Figure 23). Tie bars supposedly do not transfer any wheel load.

Figure 23: Schematic diagram showing location of tie bar in pavement crosssection

Design principle

The tie bars are designed to withstand tensile stresses, the maximum tensile force in the tie bar is made equal to the force required to overcome frictional force between the bottom of the adjoining pavement slab and the soil subgrade. Estimation of spacing and length of tie bar is explained in the following. Estimation of spacing of tie bar

Since the purpose of the tie bar is to tie concrete slabs together, the area of steel per unit length of longitudinal joint is obtained by equating the total friction to the total tension developed in the tie bar system (as explained in Figure 24).

Thus, (29)

where μ = co-efficient of friction between concrete slab and the sub-base, W = weight of the concrete slab per unit length (say per meter), σst = allowable working stress in tension for the steel used as tie bar, Ast= cross-sectional area of steel per unit length (say per meter). The weight of concrete slab per unit length can be written as, (30)

where, w = weight of slab per unit volume (say, cubic meter), B = width of the slab, and h = height of the slab. Thus, from Equation (29) and (30), the area of steel per unit length,

(31)

Assuming suitable diameter of tie bar, the spacing of tie bar can be found out so as to the requirement of steel per unit length. Estimation of length of tie bar Consider a single tie bar. The tensile force developed in the tie bar should not exceed the bond strength between the tie bar and the concrete, otherwise it can be pulled out of concrete. Thus, considering one end of the tie bar, (32)

where, as = cross-sectional area of one tie-bar, P = perimeter of one tie bar, l = length of tie bar inside the concrete slab, Sb = allowable bond strength between the concrete and the tie bar. Thus, total length of the tie bar, can be written as, (33)

where z = allowance due to inaccurate centering of the tie bar. Design example

A design example has been presented in the form of an Excel® sheet. Click here to download the worksheet.

Design of tie bar Input Parameters: Sr. No. Parameter 1 Slab thickness (h) 2 Slab width (B) 3 Coefficient of friction (\mu) 4 Weight of concrete per unit volume (w) 5 Alowable tensile stress of tie bars (\sigma_{st}) 7 Alowable bond stress of tie bars (S_b) 9 Diameter of tie bar (d) = 10 Allowance in length in tie bar (z) Design of tie bar: a) Spacing of tie bar Area of steel bar per meter width along the longitudinal joint A_{st}=\mu.w.B.h/\sigma_{st} The cross-sectional area of a single tie bar (a_s) Spacing of tie bar Provide a spacing = b) Length of plain Bar: Perimeter of tie bar (P) embedded length of tie bar (l) = (a_s * \sigma_{st})/(P * S_b) Deisgn lenght of the tie bar = 2*l +z Provide length of tie bar =

Value 0.33 3.5 1.5 24000 125 1.75 0.012 0.015

Unit m m N/m^3 MPa MPa m m

0.000333 0.000113 0.339999 339

m^2/m m^2 m c/c mm c/c

0.037699 0.214286 0.443571 443

m m m mm

Recapitulation





The spacing of tie bar is estimated from the area of the steel required per unit length of the longitudinal joint. The area of steel required is estimated by equating the maximum tensile force in the tie bar system with the force required to overcome frictional force between the bottom of the adjoining pavement slab and the soil subgrade. The length of the tie bar is estimated by equating the tensile force developed in a tie bar with the bond strength between the tie bar and the concrete slab.

Design of Runway, Taxiway and Apron Objective

Objective of this lecture is to introduce the basic concepts of airfield pavement design and to perceive the conceptual similarity (or differences) across various guidelines for pavement design. Introduction

The airfield pavement includes runway, taxiway, shoulder and the apron. Some of the portions of these components are identified as critical areas , and some are non-critical areas . As a general guideline, the critical areas are those portions where the aircraft speed is low, or the aircraft is in rest. More thickness is required for the pavement in critical areas. In the non-critical areas, for example, the central portion of the runway, the air craft is partly air-borne; also due to larger lateral wander the stress repetitions at a particular spot is small (FAA 2006, PCA 1995). For high speed (i.e. small time of contact), the asphalt shows higher strength (creep modulus) than slow speed movement. Thus, design thickness for non-critical areas turns out to be less than the critical areas. Same design chart is, generally, recommended for design of airfield pavements, and some factors are prescribed for adjustment of thicknesses for the individual components (i.e. runway, taxiway, shoulders and apron) of the airfield pavements. Input parameters The various parameters used in pavement design have already been discussed in the section 'design parameters' of the lecture ' various design approaches' . Similarly, the parameters involved in the airfield pavement thickness design process are the aircraft gear loading (gear configuration, wheel load, tyre pressure, lateral wander), load repetition, material properties (stiffness of individual layers, fatigue behaviour of bound layer(s)) and environmental factors (temperature, subrgade moisture). Bituminous pavement design methods

Some of the methods for design of bituminous airfield pavement structures are (i) Corps of Engineers (CoE) method (ii) Federal Aviation Administration method (FAA) and (iii) The Asphalt Institute method and are discussed briefly in the following: Corps of Engineers (CoE) method

This method was developed during the World War-II, when data on runway performance was collected and analysed for various airports (Horonjeff 1975). The total thickness above a particular layer is determined by the CBR value (in percentage) of the layer. Thus, the total thickness above the subgrade, sub-base, base etc. can be found out by knowing the CBR of the respective layers. This method is simple, but purely empirical. The total thickness, T, is estimated as:

(34)

where, α is the load repetition factor, ESWL is the equivalent single wheel load of the gear load assembly (in pounds), A is the measured tyre contact area (in inch2). The values of α is suggested for various gear assembly (single wheel, dual wheel, dual wheel tandem axle etc.) used for ESWL calculation, and number of load repetitions (Horonjeff 1975). From the known CBR values of the subgrade, sub-base, and base layer the individual thickness composition are obtained. Some theoretical basis of the CBR method was proposed later on, where the thickness of the pavement is so adjusted that the shear stress developed at a given level is equal to the allowable shear stress of that layer. U.S. Army Corps of Engineers, Naval Facilities Engineering command and Air Force Civil Engineer Support Agency have recently developed a combined manual for design of airfield pavement as Unified Facilities Criteria (UFC 2001) as a modified version of this design approach. Federal Aviation Administration (FAA) method

The FAA uses modified CBR method for design bituminous pavement (FAA 2006). As per the FAA method, the subgrade soil is categorized based on the soil classification group, drainage and frost damage conditions. Design charts are available for design of pavements for gross aircraft weight of 30,000 lbs or more, for single, dual, and dual tandem aircrafts. It is generally assumed that 95% of the gross weight of the aircraft is carried by the main landing gear and 5% is carried by the nose gear (FAA 2006). A typical pavement design is presented in Figure 25. The design life is assumed generally assumed as 20 years.

Figure 25: A schematic bituminous pavement design chart as per FAA (2006)

The following is the design procedure followed as per the FAA method (FAA 2006):   

The subgrade CBR value, gross take-off weight, main gear weight and annual departure of the individual aircrafts are used as the input to the design. Different types of materials can be used as base/ sub-base, and accordingly some variations are observed in the design thickness. The individual design charts (for various categories of aircrafts) are used to find out the total pavement thickness values for different aircrafts. The aircraft whose corresponding data results in the maximum thickness is designated as the design aircraft . Having obtained the design aircraft, the annual departure data of the individual air-crafts are converted to equivalent annual departure of the design aircraft. This is done by two sequential steps, as follows: o The departures of different aircrafts which have different gear configurations are converted to equivalent departure of the design aircraft, by multiplying with suitable factors. Various factors are suggested by the FAA (2006) using which departure of any gear configuration can be converted to departure of another gear configuration. o The departures of individual aircrafts adjusted for the gear configuration of the design aircraft are further adjusted for the wheel loads. This is done by using the following empirical formula:

(35)

where, Rd is the equivalent departure of the design aircraft, Ra is the departure of any aircraft adjusted for the gear configuration, W d is the single wheel load of main gear of the design aircraft, and W a is the single wheel load of main gear of any aircraft. 



Having obtained the equivalent departures of the design aircraft adjusted for gear configuration and wheel load, all the departures are summed up. Pavement design is performed for this total equivalent departure with the help of the specific design chart applicable for the design aircraft . From the known CBR values of the subgrade, sub-base, and base layer, the individual thickness composition are obtained. The dashed line indicates the order of progression. The asphalt thicknesses are generally specified for the critical and noncritical areas. To take care of the frost problem (freeze-thaw), first, the freezing index of the site is obtained. Predictive charts are developed to estimate the depth of frost penetration for various soil types and freezing index. Now, either of the following three approaches can be used for design purpose. o The thickness of the pavement can be made at least equal to the depth of frost penetration. o The frost susceptible subgrade soil may be replaced with non-frost susceptible soil. o Adequate pavement thickness may be provided assuming subgrade will have reduced strength during thawed condition.

A worksheet software for design of bituminous airfield pavement as per FAA can be downloaded from the FAA website The Asphalt Institute Method Asphalt Institute (MS-11 1987) recommends full depth asphalt pavement for air-field pavement (for gross aircraft weight higher than 60,000lbs). Mechanistic-empirical (M-E) method is used for design of pavement. As per the M-E design method, the two modes of structural failure are considered to be governing the pavement design. These are fatigue and rutting failure. The horizontal tensile strain below the bituminous layer and vertical strain on the subgrade are considered as causative factor for fatigue and rutting respectively. The field calibrated fatigue/ rutting equations are obtained by relating the initial critical strains and the number of repetitions it take for the failure due to fatigue or rutting. The allowable fatigue and rutting strains, for a design traffic, are obtained from the field calibrated fatigue and rutting equations. In M-E approach the thicknesses of the pavement layers (asphalt surfacing and base/ sub-base) are adjusted in such a way that the developed fatigue and rutting strains are comparable to the allowable strains. Concrete pavement design methods

Some of the methods for design of concrete airfield pavement structures are (i) Corps of Engineers (CoE) method (ii) Federal Aviation Administration method (FAA) and (iii) Portland Cement Association (PCA) method and are discussed briefly in the following: US Army Corps of Engineers (CoE) method

The CoE method uses fatigue equation developed from full scale load tests conducted during 60's and 70's (Smith et al. 2002). The current Unified Facilities Criteria (UFC 2002) considers fatigue of pavement slab due to edge stress. The edge stress is calculated as per Westergaard's analysis and assumes that load transfer joints reduces the stress by 25% ( Thuma and Lafrenz 2003 ). The method allows reduction of slab thickness if the subgrade strength is high. Federal Aviation Administration (FAA) method

The standard design curve assumes aircraft wheels are either placed parallel to or perpendicular to pavement joint. Westergaard analysis of edge stress calculation is employed. It is assumed that 25% of the edge stress is carried by the joints at the edges (FAA 2006). The following are the considerations for design of concrete pavement as per FAA (2006):



The modulus of subgrade reaction is estimated by performing plate load test on the subgrade. If the subgrade is multi-layered, or sub-base (granular or stabilized) is put over the subgrade, the effective

 

 

modulus of subgrade reaction needs to be determined. Charts are provided, similar to the Figure 19, presented in the lecture on 'various design approaches'. Same concept, as discussed above for the design of bituminous pavements, is applied to find out the design aircraft . Design charts are available, which involves the input parameters as, concrete flexural strength, effective modulus of subgrade reaction, gross weight of the design aircraft and the annual departure of the design aircraft. The cumulative fatigue damage principle is employed to arrive at the design thickness of the slab. A typical concrete pavement design chart as per FAA (2006) is schematically presented in Figure 26. The dashed line indicates the order of progression. Similar strategies are adopted, as discussed for design of bituminous pavement by FAA method, to take care of the frost penetration problem. Standard schemes are recommended (FAA 2006) for providing contraction, construction and expansion joints to the pavement. Provisions are also provided for jointed reinforced concrete pavement (JRCP) and continuously reinforced concrete pavement (CRCP).

A worksheet software for design of concrete airfield pavement as per FAA can be downloaded from the FAA website:

Figure 26: A schematic concrete pavement design chart as per FAA (2006) Portland Cement Association (PCA) method The fatigue equation of PCA method is developed from laboratory testing of fatigue behaviour of concrete beams (Smith et al. 2002, Packard 1995). The PCA method (Packard 1995) requires the (i) concrete properties (ii) effective strength of subgrade (or subgrade-sub-base combination) (iii) type of aircraft, load, and approximate frequency and (iv) type of pavement being designed, such as runway, taxiway, apron hangar etc. Use of 90 days strength test results of concrete is generally recommended for design purpose. The design procedure can be summarized as follows: 

The modulus of subgrade reaction is estimated by performing plate load test on the subgrade. If the subgrade is multi-layered, or sub-base (granular or stabilized) is put over the subgrade, the effective modulus of subgrade reaction needs to be determined. Charts are provided, similar to the Figure 19, presented in the lecture on 'various design approaches'.







From careful estimate of the present and future traffic, an appropriate safety factor is estimated for each of the aircraft types. The working stress is obtained by dividing the modulus of rupture of concrete with the safety factor chosen for individual aircrafts. Suggested values of safety factor range from 1.7 to 2.0 for critical areas and 1.4 to 1.7 for non-critical areas. From the pavement design chart, the design thickness slab thickness is estimated for a particular type of aircraft. Figure 27 presents a typical pavement design chart as PCA (1995). The cumulative fatigue damage principle is employed to arrive at the design thickness of the slab. The dashed line indicates the order of progression. This process is repeated for various aircrafts and the highest value of the thickness is chosen as the design thickness. The stresses created due to less critical aircrafts are also checked (by using the design charts in the reverse direction), and if the values are less than some specified value (350 psi), it is assumed that these do not add to the fatigue of the pavement slab.

Figure 27: A schematic concrete pavement design chart as per PCA (1995)

Recapitulation 



Some of the methods for design of bituminous airfield pavement structures are (i) Corps of Engineers (CoE) method, (ii) Federal Aviation Administration method (FAA), (iii) The Asphalt Institute method etc. The CoE and FAA methods are primarily empirical, and based on CBR value and thickness relationship. The Asphalt Institute method is mechanistic empirical, and considers fatigue and rutting failure of the pavement. Some of the methods for design of concrete airfield pavement structures are (i) Corps of Engineers (CoE) method, (ii) Federal Aviation Administration method (FAA), (iii) Portland Cement Association (PCA) method etc. These approaches estimates the flexural stresses in concrete slab which is modeled to be resting on elastic springs. The cumulative fatigue damage principle is employed to arrive at the design thickness of the slab.

Reinforced Concrete Pavementun Objectives

The objective of this lecture is to present a brief introduction to reinforced concrete pavements. Introduction

Reinforced concrete pavement could mainly be of three types: jointed reinforced concrete pavement (JRCP), continuously reinforced concrete pavement (CRCP) and pre-stressed concrete pavement. Pre-stressed concrete pavement has a very specialized application, and sometimes used for airport runway pavements. The JRCP and CRCP pavements are schematically shown in Figure-28.

(a) JRCP pavement

(b) CRCP pavement

Figure-28: Schematic diagram of JRCP and CRCP pavement (ACPA 2005) Design principle

Welded wire fabric or deformed bars or steel fibres are used for reinforcement purpose (Swanlund and Vanikar 2002, IRC:SP:46 1997). Reinforcement in the concrete slabs is mainly provided for counteracting the shrinkage (and associated cracks) due to temperature and moisture changes (PCA 1995). It does not allow crack to become wider (Austroads 2004), and thereby ingress of water and grit is prevented (IRC:101 1991). Since, the reinforcement provided in concrete pavement is not for contributing towards flexural strength, therefore, its position in the concrete slab is not important. Generally reinforcement is placed 50 mm below the surface (IRC:58 1988). The basic slab thickness is designed as per the plain cement concrete pavement design procedure 1 , however, due to provision of steel reinforcement, there is an effective increase of the slab thickness due to provision of steel reinforcement (IRC:101 1991).

The requirement of steel reinforcement can be calculated on the basis of maximum force that can overcome the frictional force between the concrete slab and the layer just below it. The shrinkage stress is maximum at the middle, and less at the edges or corners. Though theoretical equations have been developed for design of reinforcement, generally, some empirical relations are used to estimate the amount of reinforcement (Austroads 2004, PCA 1995, IRC:101 1991). For JRCP longitudinal steel reinforcement is about 0.15 to 0.25% of the cross-sectional area of the slab, for CRCP it is about 06 to 1.0% (Swanlund and Vanikar 2002, IRC:101 1991). For steel fibre reinforcement the amount of fibre used is generally 0.75

to 1.5% (IRC:SP:46 1997). Nominal transverse reinforcement is provided to control transverse cracking in both JRCP and CRCP. Recapitulation

  

There are could be three categories of reinforced concrete pavement, JRCP, CRCP and pre-stressed concrete pavement. The purpose of reinforcement is mainly to prevent shrinkage cracks and to hold the cracked portion together. Various empirical formulae have been suggested to calculate the percentage of reinforcement. The basic thickness design of pavement is designed as plain concrete pavement.

Concrete Pavement Shoulder Objectives

Objective of this lecture is to introduce various types of shoulders of concrete pavements, and their role as a part of the pavement structure. Introduction Pavement shoulders, though they are not intended to carry traffic directly, serves a number of purposes to the pavement, such as (Chakroborty and Das 2003): • Prevents intrusion of water to the pavement layers. • The shoulder is sometimes used as foot-path, or bikeway. • The shoulder carries traffic in special situations, and gives psychological comfort to the drivers, and sometimes accommodate vehicle parking. • When construction activity is going on the main carriageway, shoulder may be used as temporary driving lanes. A design traffic of 2-2.5% of the main carriageway traffic is generally used for design of concrete pavement shoulders (Ababio-Owusu et al. 2003). In this lecture, different types of shoulders of the concrete pavement are briefly introduced. Types of concrete pavement shoulder The shoulder to the concrete pavement could be unpaved or paved; again, the paved shoulders could be concrete or bituminous - they are discussed in the following. Unpaved shoulder If shoulder is unpaved, it should be well-shaped and attention should be paid to keep it in shape during maintenance operation. This is necessary for proper and quick drainage of the surface run-off. The slope is kept steeper than the main carriageway. Paved shoulder Paved shoulder prevents water percolation into the subgrade of the pavement more effectively than unpaved shoulder. Concrete shoulder The existing concrete slab can be widened (with same or lowered thickness) beyond the carriage-way width to form a paved shoulder, known as integral concrete shoulder (Austroards 2004). This is a preferred design than disjointed concrete shoulder, because this substantially reduces the stress and deflection of the carriage-way slab (Swanlund and Vanikar 2002, Benekohal et al. 1990) and shoulder drop-off failure is almost eliminated (FHWA 1990). Field investigations show that pavement with widened slab concrete shoulder exhibit little faulting and transverse cracking (Swanlund and Vanikar 2002). Bituminous shoulder

The bituminous shoulder is less expensive than concrete shoulder and provides desirable contrast between the carriageway and the shoulder. However, the joint (longitudinal) between the concrete carriageway and the shoulder is difficult to maintain. Infiltration of moisture through the joint causes fast deterioration of the asphalt shoulder (Swanlund and Vanikar 2002). Recapitulation   

The shoulder to the concrete pavement could be unpaved or paved. The paved shoulders could be bituminous or concrete type. The bituminous shoulder is cheaper than concrete one, but proper maintenance of the joint between the concrete carriageway and bituminous shoulder is difficult. Integral concrete shoulder substantially reduces the stress and deflection of the carriageway slab. Thus, there is a possibility that the overall pavement design be economical.

Composite Pavement Objective

The objective of this lecture is introduce pavement analysis and design considerations for composite pavements. Introduction

Composite pavement comprises of two or more pavement layers one of which is bituminous and the other is concrete. The concrete and bituminous pavements have their individual advantages and disadvantages which have been discussed elsewhere. It is believed that composite pavement, if properly designed, can take of the disadvantages associated with the individual types of the pavements. Though there could be composite pavements for fresh pavement constructions, most of the time, the composite pavement evolves due to rehabilitation projects where overlay is applied to the pavement. The composite pavement, thus, can be divided into two categories, (i) concrete overlay over bituminous pavement and (ii) bituminous overlay over concrete pavement. There could another category of pavement, where overlay consists of two layers, and the lower portion of the top overlay is made up of unbound granular material. This is called a sandwich pavement. Figure 29 schematically presents some examples of composite pavement.

Figure 29: Examples of different types of composite pavements General formulation for composite pavement

Since the composite pavement is a combination of bituminous, granular, concrete, and subgrade layer, one needs to combine the individual models (Ioannides and Khazanovich 1998) of these layers to develop a formulation for composite pavement. Such an analysis problem will require (i) strain-displacement relationship, (ii) constitutive law, (iii) equilibrium equation and suitable (iv) boundary conditions. Individual layers

From mechanics point of view, a composite pavement can be thought to be composed of three basic types of layers (Ioannides and Khazanovich 1998) viz. Burmister layer, subgrade layer and concrete layer . For simplicity in modeling the individual layers can be considered to be made up of homogenous, isotropic and elastic material. The basic characteristics of these layers can be mentioned as follows:

Burmister layer

The granular layer and the bituminous layer can be modelled as Burmister layer. A Burmister layer transfers the load through grain-to-grain interaction and it does not have flexural rigidity (Verstraeten 1967). It is assumed that this layer is infinite along the x and y direction and single circular loading may be considered as axi-symmetric about z axis. Subgrade layer

The subgrade can be assumed as Boussinesq half-space which is infinite along x and y direction and semi-infinite along z (depth) direction (Jumikis 1969). The subgrade may also be modelled as elastic spring, for example, as Winkler spring bed, Pasternak foundation, Kerr foundation etc. and accordingly the different solutions can be obtained. Concrete layer

The load in applied to concrete slab is resisted by bending action. The theory of plate bending is generally used in the analysis. If it is assumed that there is no deformation along the thickness of plate, then,σ z = 0 at any point across the depth. The basic assumption of thin plate (known as Kirchhoff plate ) bending theory is that the thickness of the plate (h) is small as compared to the other dimensions and thus the effect of and on bending is negligible. For detailed discussion on formulation of rigid pavement, the lecture on analysis of concrete pavement may be referred. Boundary conditions

Composite pavement is made up of a combination of these layers. Thus, having developed the governing equations of the individual layers, the analysis can be performed by using suitable boundary conditions. The boundary conditions for various interfaces may be mentioned as follows (Ioannides and Khazanovich 1998): (1) At interface between two Burmister layers

and

(for rough or smooth interface)

and

(for rough interface) ;

and

(for smooth interface)

The superscript t indicates top and b indicates bottom of the interface. The superscript i indicates the i th layer. u indicates the horizontal displacement and indicates the vertical displacement. (2) At interface between Burmister layer and rigid base

(for rough or smooth surface) (for rough interface) and

(for smooth interface)

(3) For Boussinesq half space

at

and also,

at

(4) At interface between Burmister layer and cement concrete slab

and

where,

is the vertical deflection of the plate.

(5) At interface between Burmister layer and spring bed and

, where

is the vertical stress of the spring.

(6) At interface between two plates

; provided

layer is not surface layer.

(for pure bending), where,

is the net vertical pressure.

(7) At the surface

When top surface is Busmister layer,

, if ; otherwise When top surface is cement concrete slab, , if

, p is the pressure applied on circular area Also,

, otherwise,

Design approach

As mentioned earlier, generally, the composite pavement is designed as overlay. The conventional overlay design applicable to composite pavement by (i) empirical and (ii) mechanistic approach and (iii) a special type of composite pavement as thin concrete pavement over bituminous pavement (known as ultra-thin white topping ) are discussed in the following: Overlay design by empirical approach

The pavement thickness, as new pavement design, required to extend pavement life by a given overlay design period is estimated by using deflection, estimated strain, or some serviceability criteria; and the existing thickness is discounted to a some lower value than the original by using suitable factor(s). This may be called as the effective thickness of the existing pavement. The difference (i.e. the thickness deficiency ) between these two determines the overlay thickness. Thus, (36) where, h0 = the overlay thickness (either concrete or bituminous), F = empirical conversion factor for converting bituminous thickness to concrete thickness, or, vice versa, as applicable, hd = estimated overlay thickness (either concrete or bituminous), C = factor to convert the existing pavement thickness to equivalent effective pavement thickness (either concrete or bituminous), n is a factor which takes care of the bonding condition of the overlay. These factors are estimated from traffic data, or, by performing non-destructive testing (NDT), or, are derived from experience. The recommendations in Asphalt Institute (1983), AASHTO (1993), FAA (2006) etc., for estimating the bituminous overlay thickness over existing concrete pavement, are variants of this principle. Overlay design by mechanistic approach

As per this approach, the composite pavement with some assumed overlay thickness is analysed mechanistically (as explained in the formulation). The critical stress-strain parameters at the critical locations are obtained, and the overlay thickness is adjusted so that the values are less than or equal to the allowable. For performing this exercise, the engineering parameters of the existing pavement layers (thickness, modulus, Poisson's ratio) needs to be known, and can be obtained through non-destructive testing. The engineering properties of the proposed overlay (modulus, Poisson's ratio) can be obtained from laboratory study. Some guidelines (FAA 2006, NCHRP 2005) recommend similar procedure for estimating the concrete overlay thickness over existing bituminous pavement. As a simplified approach, it is suggested that, the whole bituminous pavement can be idealized as spring foundation, and the effective modulus of subgrade reaction can be obtained from field study. Ultra-thin white topping

High strength concrete mixed with fibers are generally used as ultra-thin rigid overlay over existing bituminous pavement and is known as ultra-thin white-topping (UTW). The UTW pavement system is analysed as a three-layer model (UTW layer, bituminous layer and a base layer with equivalent modulus) assuming a degree of bonding between the pavement layers (Murison and Smith 2002). Bonding in UTW makes the two layers (UTW and bituminous layer)

behave as a monolithic unit and share the load. The neutral axis in concrete shifts from the middle of concrete layer towards its bottom (refer Figure-30). Due to this shifting of neutral axis, the following situations develop:  

Stress at the bottom of concrete layer reduces. Thus, the concrete layer can be made significantly thinner for the same loading as compared to a conventional white-topping overlay which generally has no or partial bond to the underlying bituminous layer (Goel and Das 2004). This however increases the corner stress at the top of the concrete layer. This may result in a possibility that the corner stress becomes critical than edge stress. In such a situation, the corner stress can be reduced by providing adequately thick bituminous layer as support to the UTW layer (Murison and Smith 2002).

Figure 30: Stress distribution in bonded and unbonded layers (Goel and Das 2004) Closing remarks

Composite pavements have recorded excellent to poor performance in different types of projects employing various combinations (NCHRP 2001, 2005). Design and construction of composite pavements need more precision and understanding. Recapitulation  

Composite pavement comprises of two or more pavement layers one of which is bituminous and the other is concrete. Composite pavement generally designed as overlay on existing pavement. The overlay can be designed, by empirical approach, where the thickness deficiency is estimated by deflection testing or other NDT methods. In mechanistic based approach, the general formulation of the proposed composite pavement structure, is presented to find out the overlay thickness so that critical stress/strain parameters are within allowable limits.